Chemical solutions are used for a variety of purposes, and it is often necessary to control the concentration of a chemical solution to achieve a particular goal. For example, the concentrations of chemical solutions must sometimes be maintained in industrial processing plants. As another example, chemical cleaning apparatus often employ chemical solutions which must have certain concentrations to be most effective. Although the concentration controlling apparatus and method of the present invention can generally be used to control the concentration of any chemical solution, the present invention is primarily directed to the automatic control of the concentration of a chemical cleaning solution. Therefore, the following discussion focuses on chemical cleaning solutions and their control and even more particularly focuses on controlling the concentration of cleaning solutions used in institutional ware washing machines. However, it should again be stressed that the concentration controller of the present invention is not limited to chemical cleaning solutions nor is it limited to the ware washing setting described herein.
Certain commercial dishwashers such as those used in hotels, restaurants, hospitals and other large organizations have a reservoir in which dishwashing water containing detergent is stored. In use of such dishwashers, this water is pumped from the reservoir to wash the dishes during a washing cycle and the water in the reservoir is not fully changed at the end of washing each load of dishes, but is used again for the next load of dishes. Further, such dishwashers have a rinsing cycle in which fresh water is delivered to rinse them and the runoff of fresh water passes through the reservoir and causes a partial change thereof. Food soil and rinse water can cause the concentration of detergent in the wash water to drop unacceptably low.
The utilization of automatic dispenses to control the concentration of chemical solutions used in ware washing or dishwashing machines is well-known in the art. Such automatic dispensers may generally be placed in two broad categories based upon their method of controlling the amount of chemical dispensed: (1) time controlled dispensers, and (2) conductivity measurement dispensers.
Time controlled dispensers dispense solvent or chemicals for a predetermined period of time. They do not monitor the concentration of the cleaning solution to determine whether there should be more or less solvent or chemical delivered during a given cycle.
In conductivity measurement controllers, on the other hand, a conductivity cell normally monitors the amount of detergent in the wash water, and when the detergent concentration falls below a desired level, the conductivity controller produces a signal which opens a valve to add more detergent or concentrated detergent solution to the wash water. The present invention is primarily directed to appartus which measure the conductivity of the cleaning solution (wash water) and act accordingly to maintain an optimum concentration.
Prior art conductivity cells generally include a pair of bare metal electrodes immersed in the cleaning solution. For example, U.S. Pat. Nos. 3,253,741 and 3,680,070 disclose such conductivity cells. Although such systems are generally useful for their intended purposes, it is perceived that they possess several shortcomings. For example, the bare metal electrodes of electrode-type conductivity cells are subject to scumming. That is, the electrodes can be coated by a film when they are used to monitor a hard water solution, a some films can erroneously indicate an acceptable high conductivity when in fact the conductivity, and therefore the concentration of the cleaning solution is unacceptably low. Similarly, food soil in the wash water and some films can cause bare metal electrodes to indicate an artificially high conductivity. Scummed electrodes must be periodically cleaned or replaced to maintain their accuracy within acceptacle limits.
In addition, temperature compensation in electrode-based systems presents a problem. Generally a third sensor e.g, a thermistor is separately immersed in the wash water.
Also, some prior art concentrations controllers temporarily overcompensate for a low detergent condition, thereby causing the concentration to overshoot the set point.
The present invention is directed to a concentration controller which employs an electrodeless sensor. This type of controller addresses the shortcomings of the prior art concentration controllers: First, an electrodeless conductivity sensor or cell reduces maintenance since scumming of bare metal electrodes is eliminated. Secondly, the accuracy of the concentration controller is enhanced, being substantially unaffected by food soil, fluctuating water hardness or fluctuating temperatures. Thirdly, a temperature compensation transducer can be readily included as an integral part of the electrodeless cell to eliminate the need for a separate temperature transducer. Finally, a preferred controller includes means for substantially eliminating the overshoot problem discussed above.